Methods for passivating stainless steel

ABSTRACT

Methods for passivating stainless steel after acid pickling treatment in the absence of nitric acid are provided. The methods include the steps of cleaning the pickled stainless steel with an alkaline composition to obtain clean steel, activating the clean steel with an activator composition to obtain activated steel, and passivating the activated steel with a passivating composition in the absence of nitric acid. In the preferred embodiment, the activator composition contains at least one activator, the activator having a significantly higher binding affinity for iron than for chromium as evidenced by their metal complex formation constants.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/426,147 filed Nov. 14, 2002, the disclosure of whichis expressly incorporated herein by reference.

FIELD OF THE INVENTION

[0002] The present invention relates generally to methods forpassivating stainless steel, and more specifically to pickling andpassivation of low chromium steel in the absence of nitric acid.

BACKGROUND AND SUMMARY

[0003] The industry standard for pickling stainless steel is to use ahot solution of nitric acid and hydrofluoric acid. Such picklingprocesses produce hydrofluoric acid vapors and waste nitrates thatrequire special disposal procedures. Alternatives to such processes havebeen explored, including mechanical abrasion and electrolytic picklingmethods. One alternative, disclosed in U.S. Pat. No. 5,821,212, assignedto Crown Technology, Inc., to Peterson (Rinse Aid and Process forStainless Steel) involves a peroxide-based rinse additive for cleaningstainless steel following pickling with more traditional mineral acids.

[0004] Numerous methods exist in industry and in the literature toimprove stainless steel's inherent corrosion resistance throughpassivation, however most involve nitric acid and/or treatment withhexavalent chromium solutions. These options can be hazardous and leadto significant waste treatment costs. In response to growingenvironmental concerns, many have attempted to create passivationprocesses that do not involve nitric acid or hexavalent chromium. Citricacid based passivation protocols have received a lot of positiveattention. However, citric acid passivation has not achieved the desiredresults. Therefore, there has emerged a need for non-nitric acidpassivation methods that are also effective in connection withnon-nitric acid pickling methods.

[0005] The present invention includes methods for passivating stainlesssteel after pickling treatment in the absence of nitric acid. Themethods include the steps of cleaning the pickled stainless steel withan alkaline composition to obtain clean steel, activating the cleansteel with an activator composition to obtain activated steel, andpassivating the activated steel with a passivating composition in theabsence of nitric acid. In the preferred embodiment of this inventionthe activator composition contains at least one activator. The activatorhas a significantly higher binding affinity for iron than for chromiumas evidenced by their metal complex formation constants.

[0006] Other features and advantages of the present invention willbecome readily apparent from the following detailed description, theappended claims and the accompanying drawings. It is to be understoodthat both the foregoing general description and the following detaileddescription are exemplary and provided for purposes of explanation only,and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTIONS OF DRAWINGS

[0007]FIG. 1 is a chart showing the Auger electron spectroscopy resultsfor type 409 stainless steel treated according to this invention.

[0008]FIG. 2 is a chart showing the Auger electron spectroscopy resultsfor type 409 stainless steel treated with a molybdenum passivatoraccording to this invention.

[0009] Although the drawings represent embodiments of the presentinvention, the drawings are not necessarily to scale and certainfeatures may be exaggerated in order to better illustrate and explainthe present invention. The exemplification set out herein illustratescertain embodiments of the invention, in one or more forms, and suchexemplifications are not to be construed as limiting the scope of theinvention in any manner.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0010] For the purposes of promoting an understanding of the principlesof the invention, reference will now be made to the embodimentsillustrated in the drawings and examples and specific language will beused to describe the same. It will nevertheless be understood that nolimitation of the scope of the invention is thereby intended. Theinvention includes any alterations and further modifications in thedescribed compositions and methods and further applications of theprinciples of the invention that would normally occur to one skilled inthe art to which the invention relates.

[0011] The present invention provides processes for treating stainlesssteel in the absence of nitric acid. The processes are environmentallyfriendly, efficient, quick, effective and affordable. One advantage ofthe methods of the present invention is that they allow the use of lowchromium steel such as type 409 steel in place of more costly higherchromium stainless steel materials. Moreover, using the methods of thepresent invention, low chromium stainless steel can be treated toachieve a high chromium/iron ratio that is comparable to the oxidelayers of higher chromium, more expensive materials that have beentreated using nitric acid. The methods of the present invention areeffective to reduce surface impurities, such as titanium and aluminum,and improve corrosion protection.

[0012] In one particular embodiment, the invention provides a three-stepimmersion process including a cleaning step, an activation step, and apassivation step. In other embodiments, the three-step process can beaccomplished by spraying instead of immersion.

[0013] According to one embodiment of the present invention, stainlesssteel is first pickled using methods known in the art to obtain pickledstainless steel. In some pickling methods, the stainless steel is acidpickled in the absence of nitric acid. In some embodiments, thestainless steel is descaled. A composition and method is disclosed inU.S. Pat. No. 5,821,212, assigned to Crown Technology, Inc., thedisclosure of which is herein expressly incorporated by reference.

[0014] After pickling, the steel may be processed by annealing, coldrolling, temper rolling, formed into sheets or other objects, or otherpost-pickling processes. After such processes, the steel may bepassivated.

[0015] The methods of this invention include cleaning the pickledstainless steel with a basic composition to obtain cleaned steel. Thebasic composition may include an alkaline cleaner with various anionic,non-ionic, cationic and amphoteric surfactants for removing mill dirt,oils, grease and other soil. The cleaning step can be important becausesuch soils may prevent passivation.

[0016] In some embodiments, the steel is also electrolytically cleanedusing methods known in the art. Electrolytic cleaning removes alloyedimpurities up to several atom layers deep. This step is desirable inmills that produce titanium carbide inclusions or other impurities onthe surface of the stainless steel. Such inclusions can render the steelunpassivatable. In addition, electrolysis smoothes and brightens thesurface. Inclusions may also be removed by chemical etching, butproduces a product with inferior surface quality and is lessaesthetically pleasing.

[0017] The clean stainless steel is then treated with an activatorcomposition. The purpose of the activator is to enrich the chromiumcontent through selective reaction of the iron from the solid steelmatrix and prevent its redeposition into the passive layer. Theactivator for the activator solution is selected from a group of organicand inorganic compounds that form iron complexes with particularly highformation constants relative to their chromium analogs. Other criteriafor selecting compounds for the activator solution include the watersolubility of the ligand, the water solubility of the complex and costof the compounds.

[0018] The present invention contemplates both organic and inorganicactivators, however more organic compounds fit the above criteria. Insome cases the activator is an organic chelator compound, such ascarboxylic acids. Examples of carboxylic acids that are useful includeoxalic, tartaric, gluconic, malic, and citric acids, as well as theirwater-soluble salts. The invention contemplates any carboxylic acid thatsatisfies the aforementioned selection criteria. The invention benefitsfrom maximizing the complexing agent concentrations, and theconcentration limit for these carboxylic acid chelators is dependentonly on solubility. Common chelators such as ethylenediaminetetraaceticacid (EDTA) and nitrilotriacetic acid (NTA) can also be utilized,however they tend to be less effective than carboxylic acids since theyalso bind chromium rather strongly.

[0019] The pH of the activator composition is selected based on thecharacteristics of the activator and is chosen to maximize the bindingaffinity. The organic activators are known to bind iron more strongly athigher pHs (i.e. in their anionic, not acidic form). By working athigher pH's, a greater percentage of the molecules are neutralized insolution to their anionic form, i.e. one drives the neutralizationreaction to the right, toward completion.

[0020] In other embodiments, the activator is inorganic. Fluoride is adesirable activator because it is known to selectively dissolve ironfrom stainless steel substrates. Fluoride is an effective activator inboth alkaline and acidic media, but an acidic pH tends to be moredesirable.

[0021] The methods of this invention also include passivating theactivated stainless steel. Passivation is defined as the chromium/ironratio in the surface oxide layer of the stainless steel. Higherchromium/iron ratios imply better passivation, which translates tobetter corrosion resistance.

[0022] The passivator section is a blend of oxidizing and phosphatingtechnologies. In one embodiment, the passivation composition isphosphoric acid based with sodium fluoride and a ferrous/ferric iron“redox buffer” to maintain the electrochemical potential within thepassive zone of the anodic polarization curve, characteristic of thestainless alloy (typically between 500-600 mV vs. Silver/Silver Chloride(Ag/AgCl)). The redox potential of the bath can be maintained by anysuitable oxidizer, however hydrogen peroxide is preferred due toenvironmental and cost considerations. The pH is another importantfactor, and can maintain by small additions of phosphoric acid. Theoptimal working pH is approximately 2.0-2.3.

[0023] As an acidic mixture, the fluoride in the bath activates, or“creates a chemically reactive surface” on the steel. Fluoride isthought to selectively dissolve iron from stainless steel surfaces,thereby improving the Chromium/Iron (Cr/Fe) ratio. In addition, as theacid in the mixture reacts with the steel, a small pH gradient is formedaway from the surface. The pH is slightly higher at the surface of thesteel, causing some insoluble phosphates to bond with surface,effectively building the oxide layer and providing additional corrosionprotection. Furthermore, since the bath is redox controlled in thepassive zone of the anodic polarization curve, significant metal lossesare never realized because the surface will immediately oxidize, orpassivate.

[0024] In one embodiment, the passivation bath includes about 25 g/L 85%w/w (“weight-to-weight”) O-phosphoric acid, 10 g/L sodium fluoride, 2.45g/L 50% w/w hydrogen peroxide, and 2 g/L dissolved iron. The source ofiron can be from any number of iron salts, but most preferably ferrouschloride. Other iron sources include, but are not limited to, ferricchloride, ferrous sulfate, and ferrous gluconate.

[0025] In still another embodiment, the invention provides a molybdenumpassivator that provides superior results in the absence of nitric acid.The molybdenum passivator can be used with both organic and inorganicactivators. Due to the similarities in aqueous chemistry betweenchromium and molybdenum, it is believed that enhanced molybdenum to ironratios can also impart improved corrosion resistance. This is furthersupported by the fact that stainless steel alloyed with molybdenum isknown to be more corrosion resistant. Furthermore, molybdenum does notpossess the high toxicity characteristic of chromium and is thereforesafer to handle. In one embodiment, the passivation bath includes about25 g/L 85% w/w phosphoric acid, 5 g/L sodium molybdate dihydrate, 5.0g/L sodium fluoride, and 1.0 mL 50% w/w hydrogen peroxide.

[0026] The activation and passivation sections are designed to createmaximum passivity. With maximizing the Cr/Fe ratio in mind, this can beachieved by selectively attacking the iron and enriching the chromium onthe steel surface. Since chromium (II) oxide is known to be more inertto mineral acids than iron oxides, it was speculated that a dilute aciddip might selectively dissolve iron from the surface and improvecorrosion resistance. Perhaps due to the incorporation of corrosiveanions (i.e. Cl⁻, SO₄ ²⁻, etc.) into the passive layer, this did notturn out to be the case. Use of these acids lead, in almost all cases,to accelerated corrosion. There seems to be a difference of opinion inthe literature regarding the effects of these anions on corrosionresistance, but the salt spray results that were obtained were quiteclear. Interestingly, a phosphoric acid/hydrofluoric acid combinationseems to be an exception.

[0027] The following examples are provided to illustrate the presentinvention but are not intended to limit the reasonable scope thereof.

EXAMPLES

[0028] In one experiment, described below, the cleaning bath wasformulated with a surfactant, Miranol JEM 50 g/L (0.98 pounds pergallon) and 50 g/L 50% w/w caustic soda. The activator bath was preparedwith 21.5 g/L oxalic acid dihydrate and 2 g/L 50% w/w caustic soda. Thepassivation bath was formulated with 25 g/L 85% w/w phosphoric acid and11 g/L sodium fluoride, 7.1 g/L ferrous chloride tetrahydrate and 2.45g/L 50% w/w hydrogen peroxide. In another experiment, described below,the ferrous chloride was substituted with 9.93 g/L ferrous sulfateheptahydrate. In both cases the balance was water.

Example 1 Comparison to Nitric Acid Pickled High Chromium Steel

[0029] A single lot of type S40900 stainless steel was obtained from astainless steel manufacturer and cut into 3″×6″ coupons for passivationtests. The coupons were identified as Series 1, 2 and 3. Themanufacturer was chosen because its method of processing does notinvolve nitric acid and therefore has limited natural passive qualities.The alloy was chosen because it has the lowest chromium content andhence possesses the least natural corrosion resistance of all thestainless steel alloys.

[0030] A singled lot of type S40900 stainless steel, which was pickledwith a nitric acid solution, was obtained from another stainless steelmanufacturer and cut into 3″×6″ coupons. The coupons were identified asSeries 4.

[0031] Passivation baths were prepared as follows: Bath 1 50% w/wCaustic Soda 50 g/L (cleaner): 34% w/w Amphoteric sur- 50 g/L factantBath 2 Oxalic acid dihydrate 21.8 g/L (activator): 50% w/w Caustic Soda2.0 g/L (pH = 10.5-11.5) Bath 3 85% w/w Phosphoric Acid 25 g/L(passivator): Sodium Fluoride 11 g/L Ferrous Chloride Tetra- 7.1 g/Lhydrate 50% w/w Hydrogen Peroxide, dropwise → ε > 500- 600 mV vs.Ag/AgCl reference electrode. (pH = 2.0-2.3, optimally) A nitric acidpassivation control bath was prepared according to ASTM method A-380 forthis series stainless steel: Bath 4 Concentrated Nitric Acid 250 g/L(Control): Sodium Dichromate Dihy- 50 g/L drate

[0032] Baths 1 and 2 were heated on a hot plate to 140° F., while bath 3was maintained at ambient temperature. While the operating temperaturesof baths 1 and 2 are non-critical, the temperature of bath 3 cannotexceed 120° F. without precipitation occurring. The nitricacid/hexavalent chrome control bath (4) was heated and maintained at120° to 140° F.

[0033] The stainless steel coupons were then subjected to a passivationtreatment of this invention by immersion in baths 1, 2 and 3 for variousperiods of time. Similarly, control samples were exposed to passivationtreatments as described in method ASTM A-380 through immersion in baths1 and 4. The degree of passivation/corrosion protection was thenevaluated via ASTM method B117 salt spray exposure. The coupons wereexamined and rated relative to one another. The results are listed inTable 1 below. TABLE 1 Passivation Results Bath 1 Bath 2 Bath 3 Bath 4Immersion Immersion Immersion Immersion Salt Spray Coupons (cleaner)(activator) (passivator) (control) Results Series 1A 5 min 5 min 5 min —excellent Series 2A 5 min — 5 min — average Series 3A 5 min 5 min — —poor Series 4A 5 min 1 min 1 min — v. good Series 1B 5 min — — 15 min v.good Series 2B 5 min — — 30 min excellent Series 3 v. poor (untreated)Series 4 good (nitric pickled)

[0034] As the results in Table 1 demonstrate, the salt spray corrosionresistance imparted by the passivation treatment of this inventionmatches or exceeds that of the nitric acid/hexavalent chrome treatment,but at considerably shorter immersion times and without the hazardousconstituents. It should also be noted that duplicate samples weresubjected to humidity chamber corrosion tests (100% humidity, 95° F.)with identical results.

[0035] The results for this experiment are presented as relative valuesbecause the experiments were conducted using accelerated corrosionchambers (i.e. salt spray, copper accelerated acetic acid spray,humidity, etc.). Using such chambers, results can vary between runs anddo not directly correlate to corrosion in real life applications.Therefore, such results are useful to the extent they are compared toresults from the same salt spray test run. In addition, the resultsobtained from one chamber to the next (even when trying to meticulouslyduplicate the parameters) are rarely equivalent, again except inrelation to other processes; i.e. a superior process in one chambershould be superior in all chambers. Due to the relative nature of theseresults, Auger analyses of the processes were performed as described inthe next example.

Example 2 Spectroscopy Analysis

[0036] In an effort to quantify the extent of passivation afforded bythe processes of this invention, samples of the coupons from Example 1were subjected to Auger electron spectroscopy analysis to determine thechromium to iron ratios in the surface oxide layer. The Auger electronspectroscopy chart is shown in FIG. 1. As expected, the analysis yieldedenriched chromium levels in the oxide layer, and chromium to iron ratiosof greater than one on samples that were process treated for periods ofat least one minute. In addition, the results showed that the enrichmentincreased as a function of treatment time, suggesting that improvedcorrosion protection could be realized from extended immersion times.Table 2 summarizes the typical results obtained from Auger analysis ofpassivation process treated samples, as well as the average passivationobtained from the ASTM A-380 treatment (described above) and the commonnitric acid pickling process. TABLE 2 Surface Oxide CharacteristicsCr/Fe Cr/Fe Passivation Cr En- Oxide Ratio Ratio Depth richment Thick-Process (surface) (O_(max)) (Cr/Fe ≈ 1) Depth ness 5 minute 1.46 1.14149 Å >1600 Å 900 Å Passivation (series 1A) 1 minute 1.24 1.05  36 Å 362 Å  95 Å Passivation (series 4A) ASTM A-380 1.3 N/A  13 Å  26° Å N/APassivation (series 2B) Nitric 0.51 0.45 —   45 Å  71 Å Pickled (series4)

[0037] As the Auger spectroscopy results indicate, the passivationprocess of this invention imparts several corrosion resistant propertiesto this low chromium alloy. First, and most importantly, the chrome toiron ratios are significantly higher than the non-treated panels as wellas their nitric acid pickled analogs. In addition, the corrosionperformance is further augmented by the improved passivation depth (thedepth at which the chrome and iron levels begin to diverge quickly totheir base metal compositions) and chromium enrichment depth (the depthat which the chromium level remains higher than the base metal chromiumcomposition). These characteristics are important to the sample'sability to resist corrosion following a scratch or an abrasion.

Example 3 Effect of Concentration Differences

[0038] To determine the efficacy of the passivation bath (Bath 3) duringperiods of concentration fluctuations, a series of baths were preparedto simulate lean and rich conditions and evaluated versus thepassivation treatment described previously. The results are shown inTable 3 below. TABLE 3 Concentration-Determined Performance TrendsPerturbation Trend Reason 2× Phosphoric Acid Moderately negativeInterference color patterns ½ Phosphoric Acid Slightly negativeCorrosion performance No Sodium Fluoride Slightly negative Corrosionperformance ½ Sodium Fluoride Slightly negative Corrosion performance 2×Sodium Fluoride Highly negative stability issue (pH) Redox potential<500 mV Slightly negative Corrosion performance Redox potential >600 mVModerately negative Corrosion performance Ferrous Sulfate NoneEquivalent corrosion performance Slightly inferior bath life

[0039] It is important to note that the importance of the iron to thepassivation process is to act as a redox buffer. The hydrogen peroxideaddition oxidizes ferrous iron to ferric iron, which helps maintain theredox potential of the passivation bath at 500 to 600 mV vs. the Ag/AgClreference electrode. As more stainless steel is processed, ferrous ironbuilds in the passivator bath and lowers the redox potential. The redoxpotential of the bath can be easily monitored using an ORP probe that isset to trigger the addition of small volumes of hydrogen peroxide thatoxidizes the ferrous iron to ferric. In this way the redox potential ofthe passivation bath can be maintained in the optimal range.

Example 4 Preparation of Fluoride Activators

[0040] An activator composition containing the following was prepared:25 g/L 85% w/w phosphoric acid; 10 g/L sodium fluoride at pH=2.56. Asecond activator solution containing the following was prepared: 21.8g/L oxalic acid dihydrate; 10 g/L sodium fluoride with 50% w/w causticsoda added dropwise to a pH of about 10-11. Activator baths wereprepared, which contained the activator compositions of this Example.

[0041] Stainless steel coupons were treated using the activator baths ofthis Example with Baths 1 and 3 as described in Example 2. Salt spraytests showed excellent results.

Example 5 Comparison of Activators

[0042] Several organic and inorganic activators were compared using theprocesses of Example 1. An alkaline bath without any activator was alsoused as a control. The relative performance results in Table 4 show thatthe superior results of this invention are due to the activators of thisinvention and not merely the alkaline pH of the activator bath.

[0043] The fluoride activators of Example 3 were also used in theprocesses of this invention. The performance results are compared inTable 4. The fluoride activators of this invention perform better whenthe second bath is maintained at acidic pH, but good results are alsoobtained at alkaline pH.

[0044] The process of Example 1 using baths 1, 2, and 3 were performedusing the common chelating agents shown below in Table 4 in place ofoxalic acid. The fluoride activators described in Example 3 were alsocompared. The results are shown in Table 4. TABLE 4 COMPARISON OFACTIVATOR PERFORMANCE RELATIVE ACTIVATOR pH PERFORMANCE Oxalic Acid >10Excellent Tartaric Acid >10 Excellent Citric Acid >10 Excellent AlkalineBath >10 Poor without Activator Fluoride 2-3 Excellent Fluoride >10 VeryGood Fluoride 2.56 Excellent Fluoride 10 to 11 Very GoodEthylenediaamine >10 Good Tetracetic Acid Nitrilotriacetic Acid >10 Good

Example 6 Spectroscopy Analyses

[0045] Series 1A and Series 4A Coupons from Example 1 were subjected toAuger Electron Spectroscopy Analysis. Values such as chromium to ironratio, passivation depth, chromium enrichment depth and oxide thicknesswere compared to the same values for other processes. The results areshown in Table 5.

[0046] Process A relates to the best results published in the literaturefor nitric acid process, performed with high chromium stainless steel.

[0047] Process B is a non-nitric acid pickling treatment on low chromiumsteel without passivation.

[0048] Process C is low chromium stainless passivated ala ASTM A-380.

[0049] Process D is nitric acid pickled/passivated low chromium steel.

[0050] Processes E and F are non-nitric acid pickled low chromiumstainless steel, passivated according to the invention for periods ofone-minute (E) and five minutes (F). TABLE 5 PROCESS COMPARISONS Cr/FeCr/Fe Passivation Cr En- Oxide Ratio Ratio Depth richment Thick- Process(max) (at Omax) (Cr/Fe ≈ 1) Depth ness A High Cr 1.75 N/A  20 Å — N/A“Best” Published Process B Low Cr 0.30 0.30 — —  65 Å non-Nitric Acidtreatment C ASTM 1.3 N/A  13 Å N/A A-380 D Nitric 0.51 0.45 —   45 Å  71Å Pickled (industrial) E Low Cr 1.46 1.14 149 Å >1600 Å 900 Å 5 minutePassivation F Low Cr 1.24 1.05  36 Å  362 Å  95 Å 1 minute Passivation

Example 7 Molybdenum Passivator

[0051] The protocol of Experiment 1 was repeated with the followingbaths: Bath 1 (cleaner): 50% w/w Caustic Soda 50 g/L 34% w/w Amphotericsurfactant 50 g/L Bath 2 (activator): 85% Phosphoric acid 25 g/L Sodiumfluoride 10 g/L at pH = 2.56. Bath 3 (passivator): 85% Phosphoric Acid25 g/L Sodium Fluoride 5 g/L Sodium molybdate 5 g/L 50% w/w HydrogenPeroxide 1.0 mL → ε > 640 mV vs. Ag/AgCl reference electrode at pH =1.7.

[0052] The use of the molybdenum passivator yielded superior results.The salt spray performance of the molybdenum passivated samples wassuperior to the prior art treated samples. Samples were also subjectedto Auger electron spectroscopy analysis to determine the molybdenum toiron ratios in the surface oxide layer. The analysis yielded enrichedmolybdenum levels in the oxide layer, and molybdenum to iron ratios ofgreater than nine on samples that were process treated for periods of atleast two minutes. The Auger electron spectroscopy chart is shown inFIG. 2.

[0053] Passivation, or chromium/iron ratio, values for the 5-minuteimmersion of the present invention compares very favorably to the priorart processes A through D. Typically a chromium/iron ratio of 1 orgreater is desirable, particularly for low chromium stainless steel.Moreover, the passivation depth for the 5-minute immersion process ofthe present invention is far superior to the best process reported inthe literature, which was performed on a HIGH chromium stainless steelalloy.

[0054] The results of these experiments show that the methods of thisinvention achieve results with non-nitric acid treated low chromiumstainless steel that is equal to or superior to prior art processes.

[0055] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that thespecification, drawings and examples be considered as exemplary only,with the true scope and spirit of the invention being indicated by thefollowing claims. It should be understood that only the preferredembodiments have been shown and described and that all changes andmodifications that come within the spirit of the invention are desiredto be protected.

1. A method for passivating stainless steel after acid picklingtreatment in the absence of nitric acid, the method comprising the stepsof: cleaning the pickled stainless steel with an alkaline composition toobtain cleaned steel; activating the cleaned steel with an activatorcomposition to obtain activated steel, the activator compositioncontaining at least one activator, the activator having a significantlyhigher binding affinity for iron than for chromium; and passivating theactivated steel with a passivating composition in the absence of nitricacid.
 2. The method of claim 1, wherein the activator is an organicchelating compound.
 3. The method of claim 2, wherein the activator is acarboxylic acid.
 4. The method of claim 3, wherein the activator is oneof oxalic acid, tartaric acid, gluconic acid, citric acid, malic acid,and mixtures thereof.
 5. The method of claim 4, wherein the activator ispresent in the activator composition in an amount from about 5 g/L toabout 25 g/L.
 6. The method of claim 5, wherein the activator is presentin the activator composition in an amount of about 20 g/L.
 7. The methodof claim 3, wherein the activator composition has a pH of at least 10.8. The method of claim 7, wherein the activator composition has a pHbetween 10 to about
 11. 9. The method of claim 1, wherein the activatoris inorganic.
 10. The method of claim 9, wherein the activator includesfluoride.
 11. The method of claim 10, wherein the activator is fluoride.12. The method of claim 11, wherein the activator composition furtherincludes phosphoric acid.
 13. The method of claim 11, wherein theactivator is present in the activator composition in an amount fromabout 5 g/L to about 15 g/L.
 14. The method of claim 13, wherein theactivator is present in the activator composition in an amount of about10 g/L.
 15. The method of claim 11, wherein the activator compositionhas a pH between about 1.5 and about
 3. 16. The method of claim 11,wherein the activator composition has a pH of about 2.5.
 17. The methodof claim 10, wherein the activator composition further includes anorganic acid.
 18. The method of claim 17, wherein the organic acid isoxalic acid.
 19. The method of claim 1, wherein the passivatingcomposition has a pH of about 2 and contains phosphoric acid, fluoride,iron and hydrogen peroxide.
 20. The method of claim 1, wherein thepassivating composition includes molybdenum.
 21. The method of claim 20,wherein the activator is an organic chelator compound.
 22. The method ofclaim 21, wherein the activator is a carboxylic acid.
 23. The method ofclaim 22, wherein the activator is one of oxalic acid, tartaric acid,gluconic acid, citric acid and malic acid.
 24. The method of claim 23,wherein the activator is present in the activator composition in anamount from about 5 g/L to about 25 g/L.
 25. The method of claim 24,wherein the activator is present in the activator composition in anamount of about 20 g/L.
 26. The method of claim 22, wherein theactivator composition has a pH of at least
 10. 27. The method of claim26, wherein the activator composition has a pH between 10 to about 11.28. The method of claim 20, wherein the activator is inorganic.
 29. Themethod of claim 20, wherein the activator is fluoride.
 30. The method ofclaim 29, wherein the activator composition further includes phosphoricacid.
 31. The method of claim 29, wherein the activator is present inthe activator composition in an amount from about 5 g/L to about 15 g/L.32. The method of claim 31, wherein the activator is present in theactivator composition in an amount of about 10 g/L.
 33. The method ofclaim 29, wherein the activator composition has a pH between about 1.5and about
 3. 34. The method of claim 33, wherein the activatorcomposition has a pH of about 2.5.
 35. The method of claim 20, whereinthe activator composition further includes an organic acid.
 36. Themethod of claim 35, wherein the organic acid is oxalic acid.
 37. Themethod of claim 20, wherein the passivating solution has a pH of about 2and contains phosphoric acid, fluoride, iron and hydrogen peroxide. 38.The method of claim 1, wherein the activator has higher complexformation constants for iron than for chromium.
 39. A method forpickling and passivating steel, the method comprising the steps of:pickling the steel by contacting the steel with a non-nitric acid basedpickling treatment to produce pickled steel; cleaning the pickled steelwith an alkaline cleaning solution to obtain cleaned steel; activatingthe cleaned steel with an activator composition to obtain activatedsteel, the activator composition containing at least one activator; andpassivating the activated steel by contacting the activated steel with anon-nitric acid based passivating composition.
 40. The method of claim39, wherein the steel is stainless steel.
 41. The method of claim 39,wherein the activator has a high binding affinity for iron.
 42. Themethod of claim 41, wherein the activator has a low binding affinity forchromium.
 43. The method of claim 39, wherein the passivatingcomposition includes molybdenum.